This invention relates to impact-modified monovinylidene aromatic polymer resin compositions having improved suitability for injection molding. In another aspect it relates to an injection molding process whereby injection molded articles having good combinations of physical properties and surface characteristics are obtained. In another aspect, it relates to improved injection molded articles having improved combinations of physical properties and surface characteristics.
In general, in the production of injection molded articles from an impact-modified monovinylidene aromatic resin it is necessary to have a resin having good processability, which resin, when molded into articles, produces articles which are strong, heat resistant and have a desirable surface appearance. Regarding surface appearance, particular difficulty has been encountered in developing impact-modified monovinylidene aromatic polymer resins which, when molded into articles, do not produce flow lines on the surface of the article. This problem has been particularly acute in the injection molding of complex parts such as television cabinets where there is a large surface area, sections of differing thickness and a number of rib sections presenting many channels to the flowing resins.
Kruse et al., in U.S. Pat. No. 4,187,260, teach that certain diperoxy-type initiators can be used to prepare impact-modified monovinylidene aromatic polymer compositions. It is maintained that such compositions produce molded articles having better surface characteristics. Kruse et al. teach that the molecular weight distribution which is produced using the diperoxy-type initiators results in reduced melt instability. Articles molded from the resins, therefore, have smooth, unfractured surfaces. Flow lines, however, do not result from melt instability or melt fracture and can still be a problem.
Cutter, in U.S. Pat. No. 4,137,388, discloses polystyrene resins, not reinforced by a rubber, having broad molecular weight distributions as does Japanese Patent Announcement Kokai No. 55/164,207.
It is an objective of the present invention to provide improved, impact-modified monovinylidene aromatic polymer resins. Desirably, these resins will have good melt flow properties and be able to be injection molded into articles having good combinations of physical properties and surface appearance. It would be very desirable if such resins would have a reduced tendency to produce flow lines when injection molded into articles, especially complex articles.
It is also an object of the present invention to provide an improved injection molding process whereby the flowlines otherwise visible on the surface of molded articles are eliminated or reduced. Another objective of the present invention is to provide improved injection molded articles wherein the flowlines are eliminated or reduced.